Power output and efficiency of internal combustion engines can be improved by tuning the properties of the intake and exhaust systems to take advantage of pressure waves propagating from the engine cylinders. In the exhaust system of a typical engine, exhaust gases are released into the exhaust system from the engine cylinder at high pressure. The resulting pressure wave travels at the speed of sound through the exhaust system. Part of the energy of the pressure wave is reflected back toward the cylinder in the form of a negative pressure wave when the pressure wave passes points where the air channel increases sharply in diameter. This typically occurs at a collector where exhaust pipes from the individual cylinders converge. In the intake system, the sudden intake of air during the intake stroke in four-stroke engines results in a negative wave, a portion of which is reflected toward the cylinder as a pressure wave by openings or dilations in the intake system.
If the return waves arrive at the cylinder at the appropriate time, they aid in the flow of gases in and out of the cylinder. If the exhaust system return wave arrives while the exhaust valve is still open, the negative pressure will help draw exhaust gases out of the cylinder. If the return wave strikes the exhaust port when both the exhaust valve and intake valve are open, the negative pressure aids in drawing a fresh charge of air and fuel into the cylinder. The return wave in the intake system aids in forcing the charge of air and fuel into the cylinder.
The return waves in the intake and exhaust system must be timed correctly in order to arrive at the intake and exhaust ports, respectively, at the appropriate times. In most engines, the air columns in the intake and exhaust systems are fixed. Since the speed of the return waves is substantially constant, the timing of the return waves is also constant. The engine therefore benefits from the return waves only for a small range of operating speeds where the opening of the exhaust and intake ports coincides with the return waves.
Some systems allow for manual adjustment of the length of the air column in the exhaust system in order to adjust engine operating speeds benefiting from the return wave. However, none of the prior systems provides a suitable means for accommodating the full range of operating speeds of the engine.
Accordingly, it would be an advancement in the art to provide a system and method for adapting the acoustic properties of an intake and exhaust systems according to the operating speed of the engine.